There is compelling evidence to suggest that specific classes of retinal horizontal cells utilize the inhibitory amino acid GABA as a neurotransmitter, and that GABA plays a key role in establishing the center-surround antagonism that characterizes the receptive field properties of retinal neurons. Carrier-mediated transport of GABA by horizontal cells appears to be an essential component of this process, and serves perhaps both in the uptake and release of the neurotransmitter. The advent of the whole-cell recording method has permitted such transport processes to be examined in unprecedented detail using electrophysiological techniques. The experiments described in this proposal are designed to utilize such techniques to examine the mechanisms underlying the transport of GABA by retinal horizontal cells, and include a study of the ionic mechanisms responsible for uptake, the pharmacological properties of the transport, and the modulation of transport by the neuromodulator dopamine. The question of whether carrier-mediated efflux of GABA from horizontal cells is electrogenic will also be explored. Finally, the effects of GABA on rod photoreceptors and rod bipolar cells will be examined in order to identify the cells that serve as the postsynaptic target for GABA released by horizontal cells. This research should provide a greater understanding of the role that GABA plays in the formation of the surround portion of the receptive fields of visual neurons.